The falling ice and rock overwhelmed a group of Nepalese guides who were ferrying equipment from Base Camp (elevation 5,270 meters) to Camp 1 (elevation 6,035 meters) for foreign clients. Sixteen guides died in the avalanche, making it Everest's deadliest day.

Joshua Stevens, data from U.S. Geological Survey, NASA/MITI, ASTER Science TeamThe avalanche began on Everest's western shoulder and slid onto the Khumbu icefall, a fast-moving mash of snow and ice that is like a massive frozen waterfall. Notorious among climbers for its shifting surfaces, fast-opening crevasses, and towering columns of ice (seracs), the Khumbu icefall is one of the most dangerous parts of an ascent of Everest.

On the day of the 2014 disaster, the Nepalese guides were following a route along the far northern edge of the icefall. Staying on the edge reduces the amount of time on the icefall, but being so close to the steep slopes of Everest's western shoulder increases the risk of being hit by an avalanche.

While a more central route would be less prone to avalanches, collapsing seracs are so common in the center that the "icefall doctors"—a select group of highly skilled Nepalese climbers who establish the route for most climbers each season—have often decided in recent years that the avalanche risk along the northern edge was the lesser danger.

With climbers lining up in record numbers for the chance to ascend Everest in 2015, the icefall route appears poised to change slightly. In the rendering (below), based on data from DigitalGlobe's QuickBird satellite and from ASTER, the proposed 2015 route tracks closer to the center of the icefall. (Note: the DigitalGlobe image was acquired after the ice avalanche, on April 26, 2014. The Landsat data in the image above was acquired on May 3, 2013.)

Joshua Stevens, data from US Geological Survey, NASA/MITI, ASTER Science Team While the modified route should reduce the avalanche risk, it will also expose climbers to more chaotic terrain. The subtle shadows that give the center of the icefall texture in the QuickBird image may seem innocuous from a satellite perspective, but at ground level these represent gaping crevasses and towering seracs that pose deadly challenges.

In the aftermath of the 2014 accident, some scientists and climbers have suggested that risks to climbers may be increasing as the climate changes and Everest's glaciers thin.

Jeffrey Kargel, a University of Arizona glaciologist who studies the hazards associated with Himalayan glaciers, offered some context: "As long as glaciers exist on Everest, crevasses and ice falls are always going to be a feature of the landscape. Snow avalanches, likewise, occur with or without climate change, and in fact they will surely occur after the glaciers disappear completely."

"However, with climate warming and the glaciers thinning, we can expect that ice avalanches and rock avalanches will occur in spots where previously they did not," Kargel added. "We can certainly expect the lethal or high-risk zones to shift in location, and maybe the magnitude of some dangerous processes will change, but it does not mean that there will be changes everywhere or that all changes will be for the worse. Past behavior will be a poor predictor for future behavior."

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